In recent years, non-woven fabrics comprising synthetic fibers, by making the best use of characteristics of those fibers, are extending their applications into various fields, such as clothing materials, medical materials, engineering and building materials, and materials for industrial use.
Among them, non-woven fabrics containing PTFE fibers are excellent in heat resistance, chemical resistance and abrasion resistance, and are expected to be further developed as highly functional non-woven fabrics.
Cotton-like PTFE materials being made into the non-woven fabrics are gathered PTFE fibers, and so far have been made in such manners as mentioned below:
(1) A process for producing continuous filaments and then cutting to a desired length.
The process for producing continued PTFE filaments is roughly classified into the following two processes.
(1a) An emulsion spinning method disclosed in U.S. Pat. No. 2,772, 444.
This method comprises extrusion spinning of an emulsion containing PTFE particles, a viscose binder, and the like and then sintering to obtain the filaments having a section of a given shape decided by a shape of a nozzle. Major problems of that method are such that a binder remains as a carbonaceous residual after sintering and the obtained PTFE filaments are colored in a dark brown, and that even if the carbonaceous residual is oxidized to be discolored, an original purity cannot be maintained. Further the method has a problem of increase in cost since complicated steps are employed.
(1b) A method disclosed in JP-B-22915/1961 or JP-B-8469/1973.
This method comprises stretching of fibers obtained by slitting a PTFE film to a desired width. A problem of this method is that the smaller the thickness of fibers obtained by making the slit width narrow is, the more easily the fibers are broken at the time of stretching.
PTFE fibers obtained by both methods (1a) and (1b) have such inherent properties of PTFE as a low friction coefficient and a high specific gravity, and therefore are not intermingled sufficiently with each other even if having been crimped (JP-B-22621/1975).
(2) A process for preparing PTFE fibrous powder in the form of a pulp and making a sheet-like material therefrom by paper making process (U.S. Pat. No. 3,003,912 and JP-B-15906/1969).
The method of the above-mentioned U.S. patent is to cut PTFE rod, cord or filament obtained by a paste extrusion, to a short length and to apply a shearing force to obtain fibrous PTFE powder.
JP-B-15906/1969 discloses a method for making fibers by applying a shearing force to the PTFE powder.
Any of the fibrous powder obtained by the above-mentioned methods can be made up to a sheet-like material by paper making process but cannot be made into a non-woven fabric by the use of a carding machine, needle punching machine, or the like as they are short in fiber length and in the form of a pulp.
Also as a process for producing a mixture of split yarns and other fibers, a process for making them into a cotton-like material simultaneously with a combing roll was proposed (JP-B-35093/1989). In that process, since a large amount of short fibers are produced (specification of Japanese Patent Application No. 78364/1993), there is a problem that in a step for making those webs into non-woven fabrics through needle punching method or water jet needling method, there is a large amount of short fibers which cannot be intermingled, which causes a not-negligible economical loss.
Therefore, cotton-like materials having PTFE fibers and other fibers obtained with a combing roll have been limited to thermal bonding non-woven fabrics which do not cause losses of even the short fibers by coating the surface of PTFE fibers with thermofusing resin layer or employing fibers for thermal bonding as the other fibers.
An object of the present invention is to provide cotton-like mixed materials having branched structure and/or loop structure (hereinafter may be referred to as "branched structure"), being excellent in intermingling property and comprising PTFE fibers and other fibers, and to provide non-woven fabrics produced thereof.
Another object of the present invention is to provide non-woven fabrics which exhibit particularly excellent characteristics of PTFE (heat resistance, chemical resistance, low friction property, electric insulation property, water-repelling property, mold releasing property and the like) and excellent characteristics of other fibers, and, as the case may be, offer an economical effect of lowering price thereof.
Yet another object of the present invention is to provide cotton-like mixed materials produced by simultaneously mixing PTFE fibers having the branched structure and other fibers, and non-woven fabrics produced thereof.